This invention relates to an improvement in process for polymerizing (curing) a photopolymerizable vehicle by exposure of same to U.V. radiation. "Curing" herein connotes polymerization and hardening to obtain a product that is practicaal for ordinary use and normally is tack-free.
The vehicle used herein is the binder for a film in the nature of a coating. The polymerized product is generally a clear one which optionally can be tinted in a variety of colors for the purpose of protecting and/or decorating. For convenience herein the binding vehicle alone for polymerization and such vehicle compounded with other ingredients will be referred to from time to time herein as a "coating". This coating can be a fluent, liquid phase-continuous material or a powdery mixture. It can have, if desired, colorants and fillers, in conjunction with such binding vehicle. Such coating also can have various other conventional additives such as pesticides, odorants, flow-control agents, bubble breakers, defoamers, platicizers, intercoat adhesion-promotors, and other ingredients conventional in surface coating films.
Conventional convection ovens and infrared sources have been used to cure (polymerize) binders in surface coating or decorating films and inks, often with the assistance of a catalyst in such coating deposit. More recently, ultraviolet (U.V.) wave energy curing of such binders (vehicles) have been suggested using suitable U.V. sensitizers for initiating photopolymerization at wave lengths in the U.V. spectrum that are transmittable through a quartz or other transparent window, generally such range understood as lying between about 1600 A and about 4200 A.
Typical U.V. emitters for such curing include the known low to high pressure mercury lamps which are known aand well described in the art of photochemistry. Of significance is the U.V. source known as the swirl-flow plasma arc radiation source described in U.S. Pat. No. 3,364,387. This plasma arc source provides a continuum radiation of high intensity, generally at least 350 watts to about 5000 watts per square centimeter steradian (about 1000 kilowatts to 15000 kilowatts per square foot of projected area). The wavelength of the radiation emitted by the foregoing plasma arc source is predominantly longer than 4000 A (about 70%) with the remainder (about 30%) being less than 4000 A in length. Other radiation sources can be used, such as, for example, lasers having a lasing output in the U.V. spectrum range as described in copending patent application Ser. No. 189,254, to de Souza and Buhovecky and assigned to the same assignee as the present invention. The subject matter of U.S. Pat. No. 3,364,387 and application Ser. No. 189,254 are incorporated herein by reference.
Advantages of the instant invention over prior proposals include especially economical and efficient utilization of U.V. energy, particularly that in wave lengths between about 3200 A and 4200 A to perform "cold" polymerization (curing of the vehicle at quite high speed) with attendant suppression of losses due to volatilization of the coatings' components, suppression of discoloration or degradation of the resulting deposit (which can be generally considered a film), and avoidance of shrinkage and distortion (the preservation of dimensional stability), and suppression of degradation of the substrate to which the vehicle is applied, particularly when such substrate is a plastic, or paper, or fabric.